The instant invention relates to chemical vapor deposition (CVD) apparatus and methods, and more particularly to a parylene deposition apparatus including a tapered deposition chamber which minimizes deposition chamber volume, and maximizes vapor flow over a substrate to be coated, and further including a dual vacuum outlet pumping arrangement which is effective for the rapid vacuum pump down of the vacuum chamber system.
Parylene is a general term used to describe a class of poly-p-xylylenes which are derived from a dimer having the structure: ##STR1## wherein X is typically a hydrogen, or a halogen. The most commonly used forms of parylene dimers include the following: ##STR2##
Parylene coatings are obtained from their related parylene dimers by means of a well-known vapor deposition process in which the dimer is vaporized, pyrolized, i.e. cleaved into a monomer vapor form, and fed to a deposition chamber wherein the monomer molecules deposit and polymerize onto a substrate disposed within the deposition chamber. The process occurs according to the following reaction: ##STR3##
Due to their ability to provide thin films and conform to substrates of varied geometric shapes, parylene polymers are ideally suited for use as a conformal external coating in a wide variety of fields, such as for example, in the electronics, automotive, and medical industries.
Octafluoro-[2,2]paracyclophane (Parylene AF4 dimer) is a fluorine substituted version of the above-noted parylenes and has the structure: ##STR4## It is known that parylene coatings which are derived from the AF4 dimer by the vapor deposition process have a very high melting temperature (about 500.degree. C.) and a very low dielectric constant (about 2.3). These characteristics make Parylene AF4 ideally suited for many high temperature applications, including electronic applications, and potentially as an inter-layer dielectric material in the production of semiconductor chips. The existing parylene coating systems as used with Parylene C, D, and N, typically include a vacuum chamber system comprising a vaporization chamber, a pyrolysis chamber coupled to the vaporization chamber, and a cylindrical deposition chamber coupled to the pyrolysis chamber in which the monomer vapor polymerizes and deposits onto a substrate to be coated. The coating systems further include a vacuum pump coupled to the deposition chamber for creating sub-atmospheric pressure conditions throughout the entire chamber system.
While the existing parylene deposition systems are effective in depositing parylene C, D, and N, there are unique characteristics of the AF4 molecule which prevent the existing parylene coating systems from providing sufficient deposition control, uniformity of layer thickness, material efficiency, and speed of coating to be compatible with existing semiconductor chip manufacturing technologies, semiconductor chip cost structures, and semiconductor chip manufacturing time constraints. Accordingly, there is currently presented a need for a parylene deposition system particularly suited to depositing parylene AF4 onto semiconductor wafers for use in the production of semiconductor chips.
The instant invention provides a parylene deposition system comprising a vaporization chamber, a pyrolysis chamber, a deposition bell having a frusto-conical shape, and a vacuum by-pass assembly wherein a high-conductance vacuum outlet is utilized to quickly reduce pressure in the chamber system, and a low-conductance vacuum outlet is utilized to maintain vacuum flow during the deposition procedure.
The vaporization chamber, pyrolysis chamber, post-pyrolysis chamber, and vacuum pump are located within a rectangular housing structure. A platen assembly is preferably located on a top surface of the housing, and the deposition bell is received and secured over the platen assembly to form a deposition chamber. The deposition bell and its associated inlet and outlet pipes and are formed as a single removable unit, and are arranged so as to provide a mating engagement of the deposition bell with the top of the platen and a mating engagement of the inlet and outlet pipes with corresponding fittings on the top of the housing. The removable deposition bell facilitates access to the platen assembly for the placement and removal of wafers onto the platen.
The deposition bell is tapered in shape having a smaller dimension inlet end and a larger dimension outlet end. The outlet end of the deposition bell is defined by a rim which is received in mating engagement with the supporting surface of the platen to define a tapered deposition chamber. The tapered shape of the deposition bell minimizes deposition chamber volume and maximizes vapor flow over the surface of the wafer supported on the platen adjacent to the outlet end of the chamber.
In order to draw a vacuum flow through the deposition bell from the inlet end to the outlet end, the vacuum pump is coupled to a plurality of outlets in the rim by means of a distribution manifold. A first, smaller diameter, outlet pipe is connected to the manifold in a predetermined location, and the plurality of outlet openings are distributed around the periphery of the rim in a distribution pattern intended to equalize the vacuum flow throughout the chamber. While the instant arrangement is effective for creating a desired deposition flow, the flow area is too small to permit rapid pump-down of the system. Accordingly, the deposition bell is further provided with a vacuum by-pass arrangement to speed vacuum cycle time. In this regard, the inlet of the deposition bell is connected to a second, larger diameter outlet pipe, which is provided with a by-pass valve adjacent the deposition bell inlet. The smaller diameter outlet pipe merges with the larger diameter outlet pipe which is coupled to the vacuum pump. In operation, by-pass valve remains open, wherein initial pump-down of the chamber system would be accomplished through both the first and second outlet pipes. As soon as a desired operating pressure is achieved, the by-pass valve is closed, and the desired pressure is maintained solely by the first small diameter outlet pipe connected to the distribution manifold.
Accordingly, among the objects of the instant invention are: the provision of a parylene deposition apparatus effective for quick and efficient deposition of Parylene AF4 onto silicon wafers in the production of semiconductor chips; the provision of a parylene deposition apparatus including a tapered shaped deposition chamber which minimizes deposition chamber volume and maximizes flow of vapor over the surface of the wafer; the provision of a parylene deposition apparatus including a vacuum by-pass assembly wherein a high-conductance vacuum tube is utilized to quickly reduce pressure in the chamber system and a low-conductance vacuum tube is utilized to maintain vacuum flow during the deposition procedure; and the provision of further means for the fast, efficient, and cost effective deposition of Parylene Af4 onto the surface of a silicon wafer.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.